Electrical device casing materials

ABSTRACT

An electrical device; e.g., a transformer, with a fluctuating magnetic field, contained within a casing comprised of steel which has a resistivity in excess of about 65 microhm-cm. at room temperature and a permeability of less than about 1.3 at 200 oersteds after a cold reduction of up to 65 percent. The steel consists essentially of 0.12-0.25% C. 14-15.5% Mn, up to 0.5% Si, 16.5-18.0% Cr, 1.00-1.75% Ni, 0.32-0.40% N, balance Fe and incidental impurities.

United States Patent Inventor George P. Seitanlkls Natrona Heights, Pa.

Aug. 11, 1969 Dec. 21 1971 Allegheny Ludlum Steel Corporation Pittsburgh, Pa.

Appl. No. Filed Patented Assignee ELECTRICAL DEVICE CASING MATERIALS 2 Claims, No Drawings [56] References Cited UNITED STATES PATENTS 2,138,754 11/1938 Andrus et al. 336/90 OTHER REFERENCES Carney, High-Nitrogen Steels: New Stainless Family Steel,Vol. 137,No. 19, pp. 138, 140, 144(1955) Primary Examiner--L. Dewayne Rutledge Assistant Examiner-J. Davis Attorneys-Richard A. Speer, Vincent G. Giola and Howard R. Berkenstock ABSTRACT: An electrical device; e. g., a transformer, with a fluctuating magnetic field, contained within a casing comprised of steel which has a resistivity in excess of about 65 microhm-cm. at room temperature and a permeability of less than about 1.3 at 200 oersteds after a cold reduction of up to 65 percent. The steel consists essentially of 0.12-0.25% C. l4-l5.5% Mn, up to 0.5% Si, l6.5-l8.0% Cr, LOO-1.75% Ni, 0.320.40% N, balance Fe and incidental impurities.

are substantially nonmagnetic at cold reductions of up to 65 ELECTRICAL DEVICE CASING MATERIALS The present invention relates to electrical devices with a fluctuating magnetic field contained within casings comprised of steel which does not become excessively heated during operation of the device and more particularly to transformers contained within casings comprised of steel which does not become excessively heated during operation of the transformer.

An improper choice of easing material can reduce the efficiency of an electrical device with a fluctuating magnetic field; i.e., an AC device or a DC device with a nonsteady current flow. Poorly chosen materials become excessively heated causing an increase in the ambient temperature of the device. This increase in ambient temperature results in higher energy losses, thereby diminishing the devices performance.

Choice of easing materials is especially important in large transformers which operate'at high current levels; e.g., large power and distribution transformers. Because of this, the present invention will be described with particular reference to transformer embodiments. It is, however, emphasized that the concepts embraced by the invention are equally applicable to all electrical devices with a fluctuating magnetic field.

The excessive heating referred to above is caused by two factors. The first and most significant factor is magnetic induction which induces a voltage in the material. It can be expressed by-the equation P=EI or by the equation P=I R since E=IR. The equation P=IR shows that heating is influenced by the second power of current. The second factor is the eddy currents whose effect is limited by the resistivity of the material.

Prior to the present invention, transformer casings; i.e., transformer covers, were comprised of alloys which were presumed to be nonmagnetic, such as AlSl type 304 stainless steel and certain manganese-nickel-iron alloys; e.g., alloys with 10.5-12.5 percent manganese, 7-8.5 percent nickel, balance iron and incidental impurities. l have discovered that those alloys are not reliably nonmagnetic and are not suitable casings for large transformers; i.e., transformers operating at currents in excess of 10,000 amperes, as they form ferromagnetic martensite when cold worked.

It is accordingly an object of this invention to provide electrical devices with a fluctuating magnetic field contained within reliably nonmagnetic steel casing materials.

It is an additional object of this invention to provide transformers contained within reliably nonmagnetic steel casing materials.

The article of this invention is an electrical device with a fluctuating magnetic field contained within a casing comprised of steel. The steel has a resistivity in excess of 65 microhm-cm., preferably in excess of 130 microhm-cm., at room temperature; i.e., 68 F., and is substantially nonmagnetic; i.e., the steel has a permeability of less than 1.3, preferably 1.1, at 200 1-1 (oersteds), at cold reductions of up to 65 percent.

Illustrative casing materials which meet the requirements of this invention are given below in table 1.

percent A1, balance iron and incidental impurities. It has a resistivity of about 145 microhm-cm.

A number of cold rolling tests were conducted to demonstrate the difference between the casing materials of this invention and those used in the past. Cold rolled were an AlSl type 304 alloy, a manganese-nickel-iron alloy of the type described above and an alloy which meets the requirements of this invention. Typical analyses for these alloys is given below in table 11.

Alloy D is the type 304 alloy, alloy E is the manganese-nickeliron alloy and alloy F is the alloy which meets the requirements of the invention.

Found below in table 111 are the results of the cold rolling tests.

TABLE 111 Alloy D Alloy E Alloy F 1: Cold (Permeability (Permeability (Permeability Reduction At 200 11) At 200 H) At 200 H) The dilTerence in the casing materials of this invention and those used in the past is readily seen from table [11. A study of the results reveals that only alloy F remained substantially nonmagnetic at cold reductions of up to about 65 percent. Its permeability was less than 1.10 at 200 H (oersteds) after a cold reduction of 65 percent. For comparison purposes the permeability of air is 1.00. Alloys D and E became somewhat ferromagnetic due to the formation of martensite. Although the permeability of alloy E is tolerable at low currents and magnetic fields, it is not tolerable at the very high currents and magnetic fields found in the large transformers used today; i.e., transformers of 600 kv. and greater. In these large transformers small amounts of ferromagnetism can have a strong influence on case heating. The effect of cold working, exemplified by the above-described tests, is compounded by the fact that casings generally undergo the most severe cold working in the area which surrounds the bushings, the area most TABLE I Composition Fe and inciden- 0 Mn S1 Cr N1 N, Mo A1 tallmpurlties Alloy A 0.12-0.25 14-155 0.5 16.5-18.0 1.00-1. 75 0.32-0.40 Balance. B,- 0.08 2.00 1.00 16.0-18.0 10.0-14.0 2.0-3.0 o.

C-..-- 0.15-1.t 20-40 10.0 7-16 Do.

The alloys of table 1 differ from the previously used casing materials insofar as they are reliably nonmagnetic; i.e., they percent. Additionally, they minimize the heating caused by eddy currents since they each have a resistivity in excess of 65 microhm-cm. at room temperature. Alloy C is preferred over alloys A and B as it has a resistivity at room temperature in excess of 130 microhm-cm. A typical alloy within the alloy C range consists of 0.9-0.95 percent C, 25-26 percent Mn, 9-10 It will be apparent to those skilled in the art that the novel principles of the invention disclosed herein in connection with specific examples thereof will suggest various other modifications and applications of the same. It is accordingly desired that in construing the breadth of the appended claims they shall not be limited to the specific examples of the invention described herein.

lclaim:

0.5 percent Si, 16.5-l8.0 percent Cr., 1.00-1.75 percent Ni, 0.32-0.40 percent N, balance Fe and incidental impurities.

2. An article according to claim 1 wherein said electrical device is a transformer.

i i l 

1. An electrical device with a fluctuating magnetic field, contained within a casing comprised of steel which has a resistivity in excess of about 65 microhm-cm. at room temperature and a permeability of less than about 1.3 at 200 oersteds after a cold reduction of up to 65 percent; said steel consisting essentially of 0.12- 0.25 percent C. 14- 15.5 percent Mn, up to 0.5 percent Si, 16.5- 18.0 percent Cr., 1.00- 1.75 percent Ni, 0.32- 0.40 percent N, balance Fe and incidental impurities.
 2. An article according to claim 1 wherein said electrical device is a transformer. 